In recent years, since variable information can be easily printed from electronic data, an electrophotographic system has been increasingly employed in an on-demand printing (POD) market which is a light printing field. In POD, it has been required that printing be performed on a recording material other than recording materials (sheets) used for image formation of the electrophotographic system in the past. As a particular example of such a recording material, for example, thick paper used as a cover sheet of a magazine, paper for advertising poster, gloss paper used as paper having high quality feeling, and coated paper used as a robust card coated with a wax or a poly(lactic acid) latex may be mentioned.
In association with the above requirement, further improvement in performance of a magnetic carrier used for a two-component development method has been desired. In recent years, in order to decrease the specific gravity of the magnetic carrier and to control the resistance thereof, it has been performed that a resin is filled in voids of a porous magnetic core particle.
PTL 1 has disclosed a resin-filled type magnetic carrier which achieves a decrease in specific gravity by filling voids of a porous magnetic core particle with a silicone resin. However, although the durability of the magnetic carrier disclosed in PTL 1 can be improved by the decrease in specific gravity, development properties are inferior, and an image defect, such as a white spot, at a boundary between a halftone image and a solid image may arise in some cases. In addition, in order to compensate for insufficient development properties, when Vpp (peak-to-peak voltage) of a developing bias, which is an alternating bias voltage, is set high, charge is injected into an electrostatic image through a magnetic carrier by a developer support, and an electrostatic latent image support may have the same potential as that of the electrostatic image, that is, so-called leakage may occur in some cases.
In order to simultaneously achieve improvement in development properties and suppression of leakage, in PTL 2, a magnetic carrier has been disclosed in which a resin portion of a surface of a resin-filled type magnetic carrier particle is optimally distributed, and an electric field strength immediately before a porous magnetic core particle is electrically broken down is specified. However, when printing is performed on many pieces of glossy paper by a developer using the resin-filled type magnetic carrier disclosed in PTL 2, although an image having sufficient glossiness can be obtained at an early stage, the glossiness of image may be decreased with time in some cases. The reason for this is believed that a silicone resin contained in the magnetic carrier is scraped off as described later. Although such a decrease in glossiness causes not any particular problems when intra-office documents and drawings are printed, the decrease in glossiness becomes a problem when high glossiness is required as in the case of POD.
PTL 3 has disclosed a carrier in which an acrylic resin coats a surface of a filled core particle formed by filling pores of a porous magnetic core particle with a silicone resin. However, since the silicone resin and the acrylic resin are not well compatible with each other, the surface of the magnetic carrier can only be sparsely coated with the acrylic resin. As a result, since the silicone resin is partially exposed, when many sheets are printed, the silicone resin will be scraped off, and the glossiness is decreased with time. In addition, since a uniform coating layer is not formed on the surface of the magnetic carrier, a locally low resistance portion is present on the surface of the magnetic carrier, and leakage may occur in some cases. Furthermore, since areas in which the silicone resin is exposed and areas which are coated with the acrylic resin are present at the same time on the surface of the magnetic carrier, the charge distribution of toner becomes broad, and as a result, fogging and density unevenness may occur in some cases.
As described above, it has been desired to obtain a magnetic carrier which achieves high glossiness as required in the case of POD and which also satisfies other performance necessary for the magnetic carrier.